Method of drying water-laid fibrous materials



United States Patent Ofifice 2,874,617. Patented Feb. 24, 1959 METHOD or DRYING WATER-LAID FIBROUS MATERIALS LewisW. Eckert, Lancaster Township, Lancaster County, Pa., assignor to Armstrong Cork Company, Lancaster, Pa., a corporation of Pennsylvania No Drawing. Application June 30, 1955 Serial N- 519,256

13 Claims. CI. 92-40 screen from a dilute slurry of the fibers in water. A'

small amount of flour or other binder may be incorporated into the furnish in some instances. The mass is consolidated to a desired thickness, water is removed by suction and pressure, and the dewatered web, still containing a large amount of water due to the porous nature of the product, is delivered to a drier where heat is applied to remove residual moisture. found that, when drying fiberboards in a conventional flame heated drier, there is a tendency for the surface of the board to be discolored upon the application of relatively high temperatures to the surface of the board to facilitate the drying operation. As a consequence, the boards have been moved through the drier at a relatively slow speed and lower temperatures have been applied in order to'efiect the necessary drying of the product without discoloration; If it were possible to apply higher temperatures, the board could be moved through the drier faster and greater production could be achieved since the driers are the slowest unit in a conventional board-forming mill.

It is an object of the present invention to provide a.

method of drying which will permit the application of higher temperatures to the surface of fiberboards and the like without the disadvantage of objectionable discoloration of the surface. I

While the discoloration'problem is particularly acute in wood fiberboards, a similar problem exists in mineral fiber acoustical material fabrication where the product is made from mineral Wool fibers dispersed in water and bound with starch or similar organic binding material. This product contains a large amount of water when delivered from the forming machine, and drying consequentlyis a major factor in the commercial production of theproduct. The product is disadvantageously discolored on the surface during drying at high temperatures, due probably to carmelizing or other degradation of the starch binder. The fibers themselves are not particularly susceptible to discoloration, since they are formed from mineral materials and are heat-resistant.

. A further object of the invention is to provide a method of drying water-laid fibrous materials containing carbonaceous binders subject to discoloration, to permit the application of higher temperatures than normal to the surfaces of the, product to dry them without objectionable discoloration resulting,

It has been In the manufacture of wood fiber acoustical boards, there is some tendency for the surfaces of the boards to be of darker color than desired, due to the natural color of the fibers from which the board is produced. It is desirable, therefore, to elfect a bleaching action on the fibers at the surface of the board if this can be done at a minimum of expense.

Another object of the invention is to provide a method of drying a wood fiberboard which will result in improvement in the surface color of the material, due to a bleaching action which is effected during drying at elevated temperatures.

According to the invention, there is applied to the surface of a formed body of felted fibers subject to heat discoloration, such as the wood fiber furnish or the starch and mineral wool fiber composition referred to above, while in wet condition and prior to drying, a solution of boric acid which penetrates into the body only slightly below the surface thereof. The fibrous mass is then passed into a drier, and water is removed. The boric acid serves to reduce discoloration of the product upon the application of temperatures which would normally result in objectionable discoloration; and it also serves, in some instances, to bleach the fibers and improve the surface color characteristics of the final dried product.

The invention first will be described in conjunction with the preparation of a wood fiber product.

A felted fibrous blank or body may be formed essentially from wood fibers. A wood fiberboard, prepared as a water-laid web as disclosed in Scott Patent 2,642,359 is an example. The web may be formed on a conventional fiberboard-making machine, such as an Oliver forming machine, in which a, slurry of fibers suspended in water is deposited upon arotating; screen and a con tinuous felted fibrous sheet is formed from which a portion of the Water is extracted, the formed sheet being delivered over rolls to a press section, such as a Downingtown press, where additional water is removed from the sheet and it is consolidated to a desired thickness. The formed sheet containing 65% to 75% by weight of water g is delivered to a station where the upper surface of it following ingredients are combined, all parts being given by weight:

Boric acid 8.00 Sodium alginate (Keltex sold by Kelco Co.) 0.25 Water This provides an aqueous acid solution, the efiective or active component of which consists essentially of boric acid.

In the preparation of thetreating solution, the Water 7 is heatedto about F., and the powdered sodium alginate is, stirred infor about 5 to 15 minutes until a uniform distribution of it in the waterhas been obtained.

Thereafter, the powdered boric acid is "added, and the mixture is stirred for about 5 minutes. The solution is maintained at a temperature of about 140 F; in order to insure. that. the-boric acid will remain in solution. If a quantity of :boric acid greater than about 8% is to be incorporated, then the temperature of the solution should be maintained at a higher level in order to insure complete solution of the boric acid in the water.

The boric acid in heated condition is applied to the surface of'the wet fiberboard sheet. Generally, a quantity equivalent to about to-6 gramsof boric acid on adry weight basis is applied per square foot of surface of fiberboard treated and is adequate for the purposes of the present invention. However, larger quantities may be employed without deleterious results, andsomewhat smaller quantities may be used.

At thetimetheboric acid treating solution is applied, the wet board as-delivered from the Downingtown press may contain'about'65% to 75% of water, as mentioned previously. The temperature of theboard may be in the order of 100 F. The application of the heated boric acid solution to such a wet surface results is immediate interspersion of the boric acid treating solution and theiwater contained inthe board, and a slight penetration of the boric acid solution into the body of: the-board occurs. It is desirable to limittlie'penetration to a very slight extent; and if-toodeep'a penetration-is obtained; the quantity of Keltexor other thickening agent may be increased;

The surface-treated board is then delivered to adrier. The first and middle zonesmay be heated to about 450 F. to 350? F. (from entrance to exit of each zone), and the exit zone may be similarly heated to about 370 F. to 260 F. It will be understood, of course, that until-such time as substantially all ofthe water-has'been removed from the wet board the temperature of the board except at the outermost surfacewill not exceed about. 212 F; and in the inner areas, theboard temperature willbe much of water contained in the board at the time'ofdelivery;

to the drier, the length of the drier zones, and other variable factors. In'any event, the boric acid solution minimizes discoloration of the board surfaces and thus permits the useof higher drying temperatures than would'otherwise be possible. Also, there is some bleaching actionwhich occurs on the fibers through the application of the boric acid treating solution and its drying at elevated temperatures, and a lighter colored product is obtained than heretofore, using the same basic board-forming ingredients. In another embodiment'ofthe invention, a mineral wool sound-absorbing board furnishcomprising nodulated mineral wool fibers 960 pounds, pearl starch and heater flour binder 165 pounds, wax size 13 pounds, and finely divided mineral filler 480 pounds, together with water sufficient to provide a total batch of about 960 gallons is formed into a sheet generally asdisclosed in Alexander Patent 2,717,538 and, prior to fissuring of the product as disclosed in that patent, a'coating of boric acid is applied to the upper surface of the board. In this embodiment, the boric acid solution, about 2.3% concentration,- is applied at room temperature, about 70 F. to 90 F. The solution may be applied by spraying it onto the surface of the wet mass of formed mineralwoolfibers and starch binder. It is a simple matter to control the quantity of applied solution. with a liquid spray; and in this instance, no Keltex or other thickening agent'is used, although if application by roll coater is desired or if too much penetration of the solution is obtained, a thickening agent may be added. With the starch boundmineral wool product referred to, about 2 grams of boric acid'per square foot of board surface treated; on a dry weight basis, have been found adequate to minimize discoloration.

The treated board is delivered to a drier and drying is atures than normallmay be applied to the surface of the board without deleterious results. The temperature WhlCh may be tolerated without objectionable discoloration will depend upon the nature of the carbonaceous binder, thequantity used in theproduct, the-typeof fibers used, and other variable factors. The boric acid treatment will in any event permit the attainment of higher surface temperatures than would be possible without the boric acid at the surface. In practice with a mineral wool product containing a starch binder, temperatures as high as.675.- to*.700?" F. in the initial:zone"and 40'0? F. to 450 F. in the final zo'ne,-'with'- a measurable board surface temperature of about 400 F. atthe instant of'exitof' the board from the drier, have b'eenattained. The dried product exhibited less-discolorationthan the same boards without the boric acid coating dried at lower temperatures requiring a'l'onger'dry-ing time.

The concentrationoftheboric acidsolution which may be used may be; varied as desired. Generally, a concentration of about 2% to 15 of boric acid in the treating solution will be found acceptable. With the lower concentrations, more solution must be applied to obtain the desired quantity of boric acid for proper results, and the additional water must be removed from the board in the drier. If more concentrated solutions than 15% boric acid are used, then the temperature necessary to maintain the boric acid in solution may be objectionably high.

The total quantity of boric acid applied to the board may vary considerably; about 4 to 15'grams per square foot of surface of fiberboard of boric acid- (on a dry weight basis) are adequate for most purposes with a product such as the board of the Scott patent. With the mineral fiber acoustical material of the Alexander patent, where the fiber furnish is principally inorganic and the component subject to discoloration is in minor proportion, much less boric acid is required and as-little as 1 /2 grams per square foot of board surface may be adequate. The upper limit onthe quantity of boric acid used is its cost and, where a water solution is used, the problem of saturation of the solution. Generally, about 15 grams per square foot of board surface are a good, practical maximum.

While sodium alginate is the preferred thickening agent, other thickening agents, such as carboxy methylcellulose and'bentonite, may be substituted'therefor, as iswellknown in the coating art. Generally, only a very small quantity of thickening agent will be used, in the order of 0.25% to 1.0% of the total weight of the treating-solution. Sufiicient should b'e'usedto maintain most of the treating solution in the upper A of the board;

As noted in the mineral wool product example, the

thickening agent may be omitted.

While the invention has been described particularly in connection with the manufacture of insulatingfiberboards and mineral wool acoustical units, it will be understood that the invention is applicable to all types of water-laid materials which are subject to discoloration upon the application of heat above a limited elevated temperature.

I claim:

1. In a method of drying a wet fibrous mass in board form to produce an open porous fibrous insulation board, the steps comprising applying directly to the upper surface of said we't fibrous board, in which the pores between the fibers are filled with water, 1.5 to 15 grams per square foot of surface on a dry weight basis of an aqueous acid solution, the effective component of which consists essentially of boric acid, to inhibit discoloration of organic material which is contained in said board and which is subject to objectionable discoloration during drying, said application of the boric acid solution to the wet board resulting in interspersion of the boric acid solutionwith the water contained in the board and pene tration of the 'boric acid solution into the body of the board below'theupper surface thereof by diffusion into the water contained in' the pores between the fibers of the board; andthereafter drying said'board by applying heat thereto which elevates the surface temperature at said treated surface above the normal discoloration temperature of said organic material, a minimum temperature of 220 F., to form an open porous insulation board, said boric acid inhibiting said objectionable heat discoloration of said organic material at the treated surface of said board.

2. In a method of drying a wet fibrous mass in board form to produce an open porous fibrous insulation board, the steps of claim 1 in which the heat applied to the board is above 260 F.

3. In a method of drying a wet fibrous mass in board form to produce an open porous fibrous insulation board, the steps of claim 1 in which the fibrous mass comprises mineral wool fibers and an organic binder subject to objectionable discoloration during drying and in which heat applied to the board is in the range between 450 F. and 700 F.

4. In a method of drying a wet fibrous mass in board form to produce an open porous fibrous insulation board, the steps of claim 1 in which the fibrous mass comprises cellulosic fibers. Y

5. In a method of drying a wet fibrous mass in board form to produce an open porous fibrous insulation board,

the steps of claim 4 in which the boric acid solution has incorporated therein a thickening agent which limits the penetration of the solution into about the upper ,4 of the board.

6. In a method of drying a wet fibrous mass in board form to produce an open porous fibrous insulation board, the steps of claim 4 in which the cellulos-ic fibers cornprise wood fibers and in which an aqueous solution of boric acid is applied in an amount equivalent to about 4 to 15 grams dry weight per square foot of surface of the board treated.

7. In a method of drying a wet fibrous mass in board form to produce an open porous fibrous insulation board,

the steps of claim 6 in which the board is dried at temperatures in the order of 450 to 260 F. from the beginning to the end of the drying cycle.

8. In a method of drying a wet fibrous mass in board form and including an organic binding material to produce an open porous fibrous insulation board, the steps comprising applying directly to the upper surface of said wet fibrous board, in which the pores between the fibers are filled with water, 1.5 to 15 grams per square foot of surface on a dry weight basis of an aqueous acid solution, the active component of which consists essentially of boric acid, to inhibit discoloration of the organic binding material which is contained in said board and which is subject to objectionable discoloration during drying, said application of the boric acid solution to the wet board resulting in interspersion of the boric acid solution with the water contained in the board and penetration of the boric acid solution into the body of the board below the upper surface thereof through diffusion into the water contained in the pores between the fibers of the board, and thereafter drying said board 'by applying heat thereto which elevates the-surface temperature at said treated surface above the normal discoloration temperature of said organic binding material, a minimum temperature of 220 F., to form an open porous insulation board, said boric acid inhibiting said objectionable heat discoloration of said organic binding material at the treated surface of said board.

9. In a method of drying a wet fibrous mass in board form to produce an open porous fibrous insulation board, the steps of claim 8 in which the organic binding material includes amylaceous material. 10. In a method of drying a wet fibrous mass inboard form to produce an open porous fibrous insulation board, the steps of claim 8 in which the fibrous material corn prises wood fibers.

11. In a method of drying a wet fibrous mass in board form to produce an open porous fibrous insulation board, the steps of claim 8 in which the fibrous material comprises mineral wool fibers.

12. In a method of drying a wet fibrous mass in board form to produce an open porous fibrous insulation board,

References Cited" in the file of this patent UNITED STATES PATENTS 1,153,970 Sutherland Sept. 21, 1915 1,738,976 Vivas Dec. 10, 1929 1,784,566 Andrews Dec. 9, 1930 FOREIGN PATENTS 16,947 Great Britain Sept. 8, 1893 

1. IN A METHOD OF DRYING A WET FIBROUS MASS IN BOARD FORM TO PRODUCE AN OPEN POROUS FIBROUS INSULATION BOARD, THE STEPS COMPRISING APPLYING DIRECTLY TO THE UPPER SURFACE OF SAID WET FIBROUS BOARD, IN WHICH THE PORES BETWEEN THE FIBERS ARE FILLED WITH WATER, 1,5 TO 1.5 GRAMS PER SQUARE FOOT OF SURFACE ON A DRY WEIGHT BASIC OF AN AQUEOUS ACID SOLUTION, THE EFFECTIVE COMPONENT OF WHICH CONSISTS ESSENTIALLY OF BORIC ACID, TO INHIBIT DISCOLORATION OF ORGANIC MATERIAL WHICH IS CONTAINED IN SAID BOARD AND WHICH IS SUBJECT TO OBJECTIONABLE DISCOLRATION DURING DRYING, SAID APPLICATION OF THE BORIC ACID SOLUTION TO THE WET BOARD RESULTING IN INTERPERSION OF THE BORIC ACID SOLUTION WITH THE WATER CONTAINED IN THE BOARD AND PENETRATION OF THE BORIC ACID SOLUTION INTO THE BODY OF THE BOARD BELOW THE UPPER SURFACE THEREOF BY DIFFUSION INTO THE WATER CONTAINED IN THE PORES BETWEEN THE FIBERS OF THE BOARD, AND THEREAFTER DRYING SAID BOARD BY APPLYING HEAT THERETO WHICH ELEVATES THE SURFACE TEMPERATURE AT SAID TREATED SURFACE ABOVE THE NORMAL DISCOLORATION TEMPERATURE OF SAID ORGANICMATERIAL, A MINIMUM TEMPERATURE OF 220* F., TO FORM AN OPEN POROUS INSULATION BOARD, SAID BORIC ACID INHIBITING SAID OBJECTIONABLE HEAT DISCOLRATION OF SAID ORGANIC MATERIAL AT THE TREATED SURFACE OF SAID BOARD. 